The invention concerns a valve actuation mechanism for an internal combustion engine on an automotive vehicle. The invention also concerns an automotive vehicle, such as a truck, equipped with such a valve actuation mechanism.
Automotive vehicles, such as trucks, often rely on an engine brake function to slow down in order, for example, to reduce wear of the friction brake pads and to prevent overheating of the friction brakes, particularly on downward slopes. It is known to perform engine brake by acting on the amount of gas present in the cylinders of the engine in two distinct phases. In a first phase, when the pistons are near a bottom dead center, one injects exhaust gases into the chambers of the cylinders so as to slow down the pistons when they move towards their high level. This is done by slightly opening at least a valve connected to an exhaust manifold, while exhaust gases are prevented to be expelled from the exhaust pipe and thereby at a certain pressure above atmospheric pressure. In the second phase, the gases which are compressed by the piston are expelled from the chamber of the cylinder when the piston is at or near its top dead center position in order to prevent an acceleration of the piston under effect of volcanic expansion of compressed gas. This is done by slightly opening a valve so as to expel gases from the cylinder. In most cases, the valve (or valves) which is (are) opened for the engine brake function is (are) a main exhaust valve. An engine brake system is described in document WO 9009514.
To perform these engine brake valves movements, also called engine brake valves lifts, the engine comprises, for each cylinder, a rocker acting, on the valves to open and close them. The rocker is acted upon by a rotating cam which has at least one lift sector to cause the lifting (opening) of the valve. If the valve is also an exhaust or an intake valve, the corresponding cam will comprise a main valve lift sector and one or several auxiliary valve lift sectors (also called main valve lift bump and auxiliary valve lift bump) When engine brake is needed, a cam follower surface of the rocker is moved in close contact with a cam of a camshaft moving the rocker so that the brake movements of the valve are obtained, when the cam follower interacts with the auxiliary valve lift sectors. In normal operating conditions of the engine, the valves should not perform these movements and the roller of the rocker is kept slightly remote from the cam so that the cam follower does not interact with the auxiliary valve lift sectors. The distance or clearance between the roller and the cam ensures that only the larger main lift sector on the cam, dedicated to the main exhaust event, causes an opening of the exhaust valve, but not one or several smaller auxiliary lift sectors dedicated to the engine brake function. This clearance is suppressed when engine brake is needed, by moving an activation piston of the rocker to make a close contact between the roller and the cam, so that engine brake dedicated lift sectors on the cam also cause an opening of the valve. An engine brake system having such valve actuation mechanism is described in WO-91/08381
In the case of a system where two valves are to be actuated, the piston can be in contact with the valves through a valve bridge.
When the engine brake valve opening(s) have been performed, a reset function is preferably to be performed. In other words, the activation piston needs to be moved towards its initial position in order to ensure that the valves are closed early enough before fuel admission, in order to prevent negative airflow by valve overlapping.
Engine brake systems generally comprise a control valve to direct pressurized control fluid pressure in a chamber adjacent to the piston to move the activation piston from its initial position to its engine brake actuation position. The control valve controls whether or not the engine brake function is activated. This control valve lets pressurized control fluid flow, at a pressure of for example 2 to 5 bars, towards each rocker as long as the engine brake function is needed. which typically lasts several seconds or tens of seconds during which the engine and the cam shaft may perform several hundreds or thousands of complete revolutions. In some systems, a check valve is provided to prevent any fluid flow out of the chamber. In some known systems, such as the one described in WO-91/08381, the check valve can nevertheless be forced to an open position, allowing the control fluid to escape the chamber when the engine brake is not needed. This is achieved when no control pressure is sent to the control valve. In known systems, there is only one control valve for several cylinders, so that it is not possible to use the control valve to empty the chamber to allow retraction of the piston, if such retraction is needed for a period of time inferior to one revolution of the camshaft.
It is known from U.S. Pat. No. 5,890,469 to use a rotating by-pass valve, housed inside the rocker and which opens or closes a fluid circuit in which a pressure raise provokes the outward movement of the piston. This by-pass valve is opened when the piston must be pushed back. The by-pass valve is rotated by a gear fixed to the rotation axis of the rocker. The solution of U.S. Pat. No. 5,890,469 is not entirely satisfying because, when a single valve engine brake technology is used, the reset valve opens the fluid circuit but at a time when no force is exerted on the piston to push it back in its normal position so that it tends to stay in a second position. At the time the valve springs exert a force on the piston, the fluid circuit is not opened, tending to prevent the piston from moving back to its first position.
It is desirable to provide a valve actuation mechanism in which, when a specific operation of the engine must he activated, the piston can be reset to its first position by using the by-pass valve especially with a single valve brake technology.
To this end, an aspect of the invention concerns a valve actuation mechanism for an internal combustion engine on an automotive vehicle, comprising rockers moved by a camshaft, each rocker being adapted to exert a valve opening force on at least a portion of a valve opening actuator of each cylinder, via an activation piston of the rocker, movable with respect to the rocker under action of a fluid pressure raise in a chamber, from a first position, to a second position, in which a cam follower of the rocker reads at least one auxiliary valve lift sector of a cam of the camshaft so as to perform an engine operating function, each rocker comprising a reset valve, adapted to release fluid pressure in the chamber. The valve actuation mechanism is characterized in that it comprises, for each rocker, a reset cam profile adapted to open the reset valve when the brake activation piston has to be moved from its second position to its first position, and in that each reset valve comprises a cam follower, adapted to drive the reset valve as a function of the movement of the reset profile.
Thanks to an aspect of the invention, the movement of the reset valve is not only dependant on the movements of the rocker and can be controlled by the movements of the reset cam or of the cam follower. This permits to open the reset valve exactly when needed and independently for each rocker. The fluid circuit is opened and the pressure reduced at the moment when the force of the springs which maintain the valves in their closed position is exerted on the activation piston. This permits to reduce the valve overlapping between the intake and the exhaust valves.
According to further aspects of the invention which are advantageous but not compulsory, such a valve actuation mechanism may incorporate one or several of the following features:                The reset profile is made on a reset cam of a camshaft of the valve actuation mechanism, and whereas the cam follower of the reset valve cooperates with the reset cam.        The reset cam is integral with or mounted on the camshaft which moves the rocker on which the reset cam is mounted.        The reset profile is made on the cam follower of the reset valve, and whereas the reset profile cooperates with the cam which is read by the cam follower of the rocker.        The cam follower of the reset valve comprises a lever arm.        The reset valve is rotatable around a longitudinal axis, and whereas the lever arm is fast in rotation with the reset valve.        The lever are is mounted on a shaft of the reset valve, said shaft protruding outside the rocker along a rotation axis of the reset valve.        Each rocker comprises a check valve movable between a first position, in which the check valve allows passage of fluid between the chamber and a fluid circuit feeding the chamber, and a second position, in which the check valve blocks passage of fluid between the chamber and the fluid circuit and whereas each reset valve is adapted to bypass the check valve and links the chamber and the fluid circuit feeding the chamber.        The reset valve comprises two parallel through holes and two parallel grooves linking the through holes with each other, and whereas the through holes communicate with the duct which bypasses the check valve when the reset valve is in its opened position.        The reset valve comprises a peripheral surface adapted to obturate the duct which bypasses the check valve when the reset valve is in its closed position.        The reset valve has a cylindrical form with a circular section.        The valve mechanism is an exhaust valve actuation mechanism.        The activation piston activates an exhaust gas recirculation function when it is in its second position.        The activation piston activates an engine brake function when it is in its second position.        The engine brake function is realized by opening two exhaust valves and whereas the valve opening force is exerted by the activation piston on the whole valve opening actuator.        The engine brake function is realized by opening one of two exhaust valves, whereas the portion of the valve opening actuator on which the valve opening force is exerted is a slider block, on which said valve is mounted and which is movable, with respect to the valve opening actuator, along the opening axis of said valve, and whereas each rocker comprises a finger adapted to exert, under rotation of the rocker, a valve opening force on the remaining portion of the valve opening actuator, on which the second valve is mounted.        The valve actuation mechanism is an intake valve actuation mechanism.        
The invention also concerns an automotive vehicle, such as a truck, comprising a valve actuation mechanism as mentioned here above.